powerpc/mce: Remove unused but set variable

Remove the unused but set variable srr1 in save_mce_event() to
fix the following GCC warning when building with 'W=1':

  arch/powerpc/kernel/mce.c:75:11: warning: variable 'srr1' set but not used

It has never been used.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>